1. Field
The present disclosure relates to a cathode active material, a cathode including the cathode active material, and a sodium secondary battery including the cathode.
2. Description of the Related Art
As the demand for portable electronic devices, such as video cameras, cellular phones, and notebook PCs which are lightweight and provide high performance has increased, numerous studies to develop high capacity and high energy density batteries for use as power supply devices in such devices have been performed.
Lithium batteries have been mainly used for commercial applications since they enable rapid charging and have energy densities per unit weight that are about 3 times higher than those of lead storage batteries, nickel-cadmium batteries, nickel hydrogen batteries, and nickel zinc batteries.
On the other hand, lithium batteries have safety limitations due to the high reactivity of lithium, and lithium is relatively expensive.
Sodium batteries have been actively studied for use as medium- to large-sized batteries for power storage system and/or in electric vehicles since they are environmentally friendly and have excellent price competitiveness compared to lithium batteries. Also, sodium batteries have high energy storage characteristics.
Sodium transition metal oxides including transition metals such as manganese (Mn), iron (Fe), nickel (Ni), cobalt (Co), vanadium (V), or chromium (Cr) may be used as cathode active materials for sodium batteries.
Sodium transition metal oxides have structural advantages such as higher theoretical capacity than polyvalent anion including phosphorous oxides such as phosphates, or fluorophosphates, and provide sufficient transfer paths for sodium ions in their crystal structures.
However, electrodes including sodium transition metal oxides have poor lifetime characteristics since they rapidly deteriorate due to transition of the crystal structure according to variations in the oxidation number of a central metal during charging or discharging, and also have a poor discharge capacity since the content of inert sodium in the crystal structure is high.
Accordingly, there remains a need for cathode active materials for sodium secondary batteries having improved discharge capacity and lifetime characteristics.